Friday, 16 August 2013

Effects of Deepwater Horizon oil spill likely to last for decades

The catastrophic explosion on the Deepwater Horizon
drilling platform allowed five million barrels of oil to gush into the
Gulf of Mexico between April and July 2010. Since then, remediation and
restoration efforts have helped to remove obvious signs of the disaster
and return affected coastal communities to their pre-spill condition.
But according to a recent study published in the academic journal PLoS ONE, the benthic, or sea-floor, environment around the disaster area still may require years to make a full recovery.

These conclusions were based on analyses of deep-sea sediment samples
collected at 68 sites throughout the Gulf. The sampling locations,
which were visited in autumn 2010, were selected because they radiate
outward along a contamination gradient stretching away from the wellhead
from which the crude oil flowed. The nearest sampling points were less
than 1km from the wellhead, while the farthest were 125km away.

Emergency crews respond to the Deepwater Horizon explosion and spill. Image courtesy of the U.S. Coast Guard.

At each site, researchers characterised the physical properties of
the benthic sediment and tested for signs of oil and other contaminants.
They also quantified the abundance and diversity of benthic organisms
commonly used as bioindicators: larger bottom-dwelling creatures known
as macrofauna, and smaller species called meiofauna. A statistical technique called a principal component analysis (PCA) was then employed to look for relationships between these ecological variables.

The analysis revealed a clear link between contaminants associated
with drilling (especially barium, polycyclic aromatic hydrocarbons, and
total petroleum hydrocarbons) and the diversity of benthic creatures.
Specifically, areas with more contaminants had very high abundances of nematodes relative to copepods,
but low overall diversities of both nematodes and copepods. The driver
of these results is unclear, but the researchers hypothesise that the
influx of organic material from the oil spill might have led to a
bacterial bloom. This would have been advantageous to bacteria-eating
nematodes, but could have been harmful to copepods and nematodes with
other diets.

To get a clearer picture of how these patterns were affected by each
sampling site’s proximity to the wellhead, the researchers used a
colour-coding scheme to label each site. Areas that had high
contamination and low biodiversity (eg, those where the spill’s
footprint was biggest) were marked in green, while areas with low
contamination and high biodiversity (the smallest footprint) were
labelled in red.

Map
showing the footprint of the Deepwater Horizon oil spill at each of 68
sampling sites visited in late 2010. Gradient runs from green (least
affected) through yellow to red (most affected). Image courtesy of Paul Montagna et al.

While the red sites tended to be in close proximity (within 3km) of
the wellhead, orange and yellow site – those with moderate footprints –
were more variable. This pattern was probably caused by an underwater
plume of oil up to 200m thick and 2km wide in some areas that helped
distribute contaminants along its path to the southwest of the wellhead.
Because of this deep-sea flow of hydrocarbons, some distant habitats
were more contaminated, while some closer ones were cleaner than
expected.

The final step of the study was to increase the extent of the
colour-coding and produce a map estimating the footprint across the
entire region, rather than just at the 68 sampled points. This was
achieved using an interpolation technique known as kriging,
which allows researchers to extrapolate from the data they have
collected to estimate values at other, non-sampled, sites. The result is
a map that covers over 70,000km2 of benthic habitat, of which 167km2 is classified as moderately contaminated and another 24km2 of which is classified as severely contaminated.

The biggest remaining question is how long the benthic community will
continue to feel the negative effects of the spill. Gulf microorganisms
will eventually be able to clean up their habitat, but this process
could take a while given the cold temperature and low nutrient levels
found in the contaminated area. In fact, given the lengthy recuperation
period previously documented for past oil spills in shallower seas, the authors predict that it may take decades to undo the damage caused by the Deepwater Horizon blowout.

Who is the "Anthrophysist"?

I am a biologist who studies the ways in which anthropogenic disturbance impacts animals (especially birds). I hope that the results of my work, and the work of other researchers like me, can help humans learn how to coexist more peacefully with wildlife. I am also interested in the role that nature has played in shaping human cultures around the world and over the centuries. Although this blog will predominantly focus on scientific research, I hope to occasionally profile some anthropological work as well, in order to better highlight the interconnectedness of humans ("anthro") and nature ("physis").